They are red, reasonably large and have no business in the main belt of asteroids, but their discovery confirms the complex conditions that prevailed while the solar system was still forming.
New research published in The Astrophysical Journal Letters describes the discovery of two extremely red main-belt asteroids. Named 203 Pompeii and 269 Justitia, the asteroids have a redder spectral signature than any other asteroid in the main belt, that highly populated asteroid belt located between the orbits of Mars and Jupiter. The new paper led Japan Space Agency (JAXA) astronomer Sunao Hasegawa.
It is important that these red asteroids resemble trans-Neptunian objects, ie objects located faaway from Neptune, the most distant planet from the Sun (without disrespect to the dwarf planet Pluto). This could mean that 203 Pompeii and 269 Justitia came out there in the Kuiper belt and then moved inward when the solar system was still young. If confirmed, the new finding shows how chaotic the conditions were at the time and that materials from different parts of the solar system would sometimes mix.
The purpose of the study was to document the distribution and composition of large asteroids in the main belt. Large asteroids, especially those more than 100 kilometers wide, probably survived the first days of the solar system. By studying these objects, scientists hoped to see what the conditions were like about 4 billion years ago.
To do so, astronomers made visible and near-infrared spectroscopic observations of the mainband using a telescopic object (IRTF) and the Astronomical Observatory of the National University of Seoul (SAO). This international collaboration included scientists from MIT, University of Hawaii‘and, Seoul National University, Kyoto University and several other institutions.
Asteroid 203 Pompeii has a diameter of 110 kilometers, and 269 Justitia is half smaller. Both have an unusually red spectrum, meaning they reflect a lot of red light. They are even redder than type D asteroids, which were previously believed to be the reddest objects in the asteroid belt.
The outer solar system is full of materials left behind after the formation of the solar system, including planetesimels (asteroids) and centaurs (icy planetesimals located between Jupiter and Neptune). These distant objects are very red and contain complex organic compounds such as methane and methanol. These compounds, viewed through a spectrograph, give the asteroid a reddish appearance. In contrast, objects in the internal solar system have scarce traces of organic material, so they tend to reflect blue light.
Asteroids 203 Pompeii and 269 Justitia “are thought to have formed near the outer edge of the solar system beyond the distant limit of organic snow and then moved into the asteroid belt during the early epoch of the formation of the solar system,” notes JAXA press release. By “organic snow line”, scientists mean a location in the solar system where methanol and methane turn to ice.
This finding suggests that some asteroids in the main belt formed in the outer solar system, and that the population of these objects is likely to exist within the main belt. A good next step would be to determine the exact proportion of this population of red asteroids. Moreover, the new study shows the main belt as a good destination for a future mission. Instead of traveling to the outer edge of the solar system for samples of Kuiper Belt objects, all we should do is send a probe to the asteroid belt, where it could study the asteroids of inner objects and those formed far, far away.